In a well known class of maskless lithography systems, a spatial light modulator (SLM) such as a Digital Micro-Mirror Device (DMD) or Grating Light Valve (GLV) or liquid crystal (LC) light valve is used for spatially modulating a beam to form an image or pattern to be printed. DMDs are SLMs in which the modulating elements are several hundred thousand microscopic mirrors arranged in a rectangular array including rows and columns. As used herein the rows and the columns in the rectangular array are defined such that the rows include more modulating elements than the columns. Each of the mirrors in the array can be individually rotated to an ON or OFF state. In the ON state, light from the light source is reflected into the optical system directing light toward the writing surface and in the OFF state, the light is directed away from the writing surface, e.g. into a light trap or heat sink.
Although DMDs are used in maskless lithography systems, they are primarily intended to be used for digital light processing projectors and rear projection televisions. The aspect ratio of the rectangular array is therefore configured for standard picture formats, e.g. television and projector screens. When employing DMDs for maskless lithography, scanning is required to accommodate imaging over an area which is typically considerably larger than an exposure area obtained from a single DMD or even from several DMDs.
Scanning is most often performed generally parallel to the column direction. Since the DMD includes a plurality of rows, scanned areas are overwritten numerous times (corresponding to the number of rows in the DMD) during each pass. Different systems and methods have been proposed to harness the plurality of rows available on the DMD for example to improve resolution, to avoid pixelation or other aliasing effects such as jagged edges in diagonal patterns, and/or to control exposure intensity.
U.S. Pat. No. 5,208,818 entitled “Laser system for recording data patterns on a planar substrate”, the contents of which is incorporated herein by reference, describes a phototool generator that uses a pulsed excimer laser to image a DMD onto a substrate. The excimer laser typically has a Gaussian or trapezoidal beam profile that results in non-uniform exposure on the substrate. The uniformity of the exposure obtained with the excimer laser is improved by using an overlap method in which each spot is exposed four times with a slight two dimensional shift between exposures, so that the resulting summation of the exposure is uniform. Overlapping or overwriting is performed by a step and repeat process where precision stages are shifted at the end of each scan. One of the undesirable features of such a step and repeat process is that it requires multiple laterally shifted scanning.
U.S. Pat. No. 6,425,669 entitled “Maskless Exposure System”, the contents of which is incorporated herein by reference, describes a photolithography system and method for writing a pattern generated from a pixel panel. The pixel panel typically includes a plurality of DMDs or optionally an LCD. In some embodiments, pixel elements from the pixel panel are alternately directed to a first site and then a second site that partially overlaps the first site. Overlapping at the image level provides for compensating for errors due to gaps between images or faulty pixels in the pixel panel. Additionally, overlapping at the pixel level is used for accommodating diagonal projections or non-linear structures. In one embodiment, overlapping is provided by a parallel prism, positioned in the light path between the pixel panel and wafer that offsets the image. A motor alternately positions the prism inside and outside the light path to alternate between the first and second site. Typically, such a system is specifically suitable for step and repeat exposure systems.
U.S. Pat. No. 6,537,738 entitled “System and method for making smooth diagonal components with a digital photolithography system”, the contents of which is incorporated herein by reference, describes a digital photolithography system for use in making smooth diagonal components (e.g., lines and edges). A DMD provides a first digital pattern for exposure onto a plurality of wafer sites. After exposure, the wafer is displaced a distance less than the site length of the wafer. The DMD then receives a second digital pattern for exposing a second plurality of pixel elements onto the plurality of sites of the subject. The exposed second plurality of pixel elements overlaps the exposed first plurality of pixel elements. This overlapping allows incremental changes to be made in the image being exposed using a step and repeat process, thereby accommodating the creation of smooth edged diagonal components. In some embodiments, the pixel panel is angled with respect of the subject and the scan direction. As the system scans, successive pixel elements are exposed slightly offset in the y direction. Typically, substantial tilt angles that achieve only moderately dense addressing are used with such a system to ease the burden of writing data handling. This in turn mandates substantial swath overlap that is created between passes. Another known disadvantage of scanning with a tilt is that tilting typically results in non-Cartesian addressing of the scanned pixels.
U.S. Pat. No. 6,903,798 entitled “Pattern Writing Apparatus and Pattern Writing Method”, the contents of which is incorporated herein by reference, describes a pattern writing apparatus for writing a pattern on a photosensitive material with a DMD. The DMD is tilted relative to the main scan direction. A center-to-center distance along the scanning direction between such two adjacent irradiation regions arranged in the main scanning direction is made equal to “a times” (a is an integer equal to or larger than 2) the pitch of writing cells on the substrate with respect to the scanning direction. This geometry determines the addressing pitch of a Cartesian addressing space. Smaller DMD rotation angles are associated with finer addressing space.
Typically, for small DMD rotations and a correspondingly finer addressing pitch, addressing uniformity becomes susceptible to optical and mechanical distortions. Furthermore, smaller DMD rotations and correspondingly finer addressing pitch in practice dictate larger “n” values. Making “n” larger does allow for higher writing speeds, but also causes “smear” (or blurring) due to the continuous motion during the active frame time. This blurring effect may be somewhat offset by turning the light source OFF before the next frame reset, at the expense of exposure energy utilization. Alternatively, higher writing speeds may be achieved while reducing the number of exposures, by using only a portion of the DMD for writing, e.g. only 192 rows of the DMD out of the 768 rows available, and the rest are permanently kept in the OFF position. This of course also results in reduced exposure energy efficiency.
U.S. Pat. No. 7,136,087 entitled “Multi-Exposure Drawing Method and Apparatus Therefore”, the contents of which is incorporated herein by reference, describes an exposure unit including a DMD that is moved at a constant velocity in a drawing direction that forms a slight angle with respect to the elements of the DMD. An exposure is provided when the DMD is moved in the drawing direction by a distance of “A+a” where “A” is a distance corresponding to an integer-multiple of a distance exposed by one element of the DMD, and “a” is a fraction of a distance exposed by one element of the DMD. The inclination angle of the drawing direction and the rate of exposure can be adjusted to obtain a desired resolution. Small DMD rotations typically require long reset intervals which results in substantial motion smear. This effect is typically minimized by turning the entire DMD to an OFF state in between any two active frames. Although the motion smear is reduced in this manner, the exposure energy efficiency is likewise reduced.
U.S. Pat. No. 7,295,362 entitled “Continuous Direct-Write Optical Lithography,” the contents of which is incorporated herein by reference, describes a lithographic method that includes illuminating a spatial light modulator with an area array of individually switchable elements and projecting an image of the spatial light modulator on a photosensitive surface of a substrate while moving the image across the surface of the substrate. While the image is moving, elements of the spatial light modulator are switched so that a pixel on the photosensitive surface receives, in serial, doses of energy from multiple elements of the spatial light modulator, thus forming a latent image on the surface. The image is blurred to enable sub-pixel resolution feature edge placement. The number of pixels exposed on the substrate surface in the cross scan direction during scanning corresponds to the number of elements available along a row of the spatial light modulator.
US Patent Application Publication No. 20060060798 entitled “Method and Apparatus for Multi-Beam Exposure,” the contents of which is incorporated herein by reference, is in the field of planographic printing and describes a multi-beam exposure method and an apparatus for recording a stable halftone expression with sharp dot edges using an FM screen when performing the exposure process with a DMD by increasing the number of dots (the number of beam spots) which can be simultaneously exposed in a direction orthogonal to the scanning direction. The apparatus includes a pixel block shifting member to divide a plurality of exposure beam spots emitted from a DMD into a plurality of blocks in the scanning direction and shifting each pixel block with respect to another pixel block in a direction orthogonal to the scanning direction for scan-exposing a space in a direction between the exposed beam spots with the exposure beams spots of another block. Typically, the block shifting member is arranged behind and proximal to a micro-lens array on the optical path in order to focus the dots onto the recording medium. The micro-lens array provides for non-overlapping exposure beam spots. Optionally, the array of separate dots is generated using an aperture array mask in place of the micro-lens array. Optionally, the separate beam spots are further split using a polarizing element into pairs of partially overlapping spots to obtain an approximately rectangular shape.